In foundry art, cores and molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resulting mixture is rammed, blown, or otherwise formed to the desired shape or pattern of the core or mold, and then cured to a solid using a catalyst, a co-reactant, and/or heat.
One group of processes which do not require heating in order to achieve curing of the cores and molds are referred to as no-bake processes. In such processes, the binder components are coated on an aggregate material during a mixing step, and then formed around the cope and drag halves of a mold. The binder components cure after a finite period of time, thus hardening the aggregate mixture in preparation for pouring of metals during casting. The elimination of the heating step has, historically, resulted in a reduction of costs in no-bake processes as compared to earlier technologies.
Several types of no-bake binders are known. Furan no-bake (FNB) binders were introduced as early as the 1950s. Furan binders are generally regarded as the first genuine no-bake binders, and they are generally easy to control and have excellent strength. However, FNBs must contend with significant raw material price swings, production limitations, and the disagreeable smell of sulfur dioxide emissions during the casting process. Phenolic-urethane no-bake (PUNB) binders have been around since the 1970s, and these binders enhanced the ease of mold (aggregate) reclamation and use of no-bake binder systems in high speed production of metal castings. However, a primary drawback of PUNBs is the comparatively high levels of odor and smoke at molding and casting due to the presence of solvents comprising volatile organic compounds. Organic ester-cured alkaline phenolic no-bake binders, meanwhile, were introduced in the early 1980s. These binder systems employ water soluble resins that have comparatively lower levels of odor emission and toxicity during molding and casting, resulting in better working conditions and increased foundry productivity.
Commonly used resins in ester-cured alkaline phenolic no-bake binder systems include phenol-formaldehyde polymer. As a result, despite the comparatively lower odor production of these systems, free formaldehyde is emitted during the molding and core making processes that use this system. As a gas, formaldehyde has a pungent, offensive odor. The Occupational Safety & Health Administration (OSHA) has established permissible exposure limits for the substance. Additionally, formaldehyde can cause irritation and burning of the eyes and nose of people exposed to it, such foundry workers. Thus, adequate control or elimination of formaldehyde emission during the use of ester-cured alkaline phenolic no-bake binder systems is desirable.
United States Patent Publication Number 2005/0250872 to Fox et al. teaches a process comprising adding a mixture of an aqueous alkaline phenolic resole resin and a liquid organic ester for the purpose of immobilizing large swaths of an aggregate, such as sand in a desert. Fox et al. further discloses that urea may be added to the resin as a scavenger that reacts with unreacted formaldehyde to allegedly remove odor caused by the same.
U.S. Pat. No. 6,559,203 to Hutchings et al., teaches a combination of furan binders and resorcinol in foundry molds. Hutchings et al. further teaches a combination of ester cured phenolic resole resin binders and resorcinol to demonstrate general improvement in long-term tensile strengths and humidity resistance of foundry cores made with the aforementioned binders.
It is notable that urea has been used in the prior art to reduce formaldehyde emissions. However, it can only be added in limited amounts, as it is a source of nitrogen so the level needs to be controlled in steel castings to avoid nitrogen related defects. Accordingly, the amounts of urea currently used are not adequate to effectively reduce formaldehyde.
The prior art fails to address the longstanding unmet need in the foundry industry for ester-cured alkaline phenolic no-bake binder systems that will not only help foundries comply with OSHA standards for worker safety and health, but also improve the working environment around the systems by reducing the offensive odors and toxicity associated with intolerable formaldehyde emission levels.
Accordingly, it would be advantageous to provide an ester-cured alkaline phenolic no-bake composition that reduces formaldehyde odor and emissions as compared to conventional ester-cured alkaline phenolic no-bake technologies, without sacrificing performance (i.e., tensile strength) in the associated molds and cores or imparting nitrogen related defects in steel castings, as described herein.